1. Field of the Invention
The invention relates to frequency modulated, continuous wave radar systems, and more specifically to linear frequency sweep generators for use in such radar systems.
2. Description of the Prior Art
Range resolution of frequency modulated continuous wave (FMCW) radar systems is a function of the linearity of the transmitted signal frequency variation as a function of time.
The frequency sweep circuits in such systems typically employ a voltage controlled oscillator (VCO) which is tuned by means of a drive voltage that is either used to bias the VCO itself or to actuate a varactor diode installed in a microwave cavity associated with the VCO.
With either of the aforementioned tuning methods, the output frequency is a non-linear function of the drive voltage. Therefore in order to provide the necessary linear frequency sweep, a non-linear drive compensation circuit or linearizer must be utilized.
The foregoing situation may be better visualized by referring to FIGS. 1(a) and 1(b).
FIG. 1(a) illustrates the desired situation. The transmitted frequency rises linearly from time t.sub.0 in a sawtooth pattern. At time t.sub.1, the signal reflected from the target is received by the radar system. The frequency of the received signal also rises linearly at the same rate as the transmitted signal. The time t.sub.1 of course, is dependent upon the distance to the target. The two signals are mixed in the radar system to provide a beat signal f.sub.B which is an indication of the distance to the target. Because of the linearity of the transmitted and received signals, the beat frequency remains constant throughout the cycle.
FIG. 1(b) illustrates a situation where the VCO is driven by a linear ramp voltage but the output of the VCO, and therefore the transmitted frequency, changes in a non-linear fashion. The delayed received signal frequency changes in the same fashion, but because of the non-linearity, the beat frequency changes throughout the cycle. Linearizers typically correct for this non-linearity by driving the VCO with a voltage ramp having an inversely curved voltage versus time characteristic.
Co-pending patent application Ser. No. 696,236 filed in the name of Richard W. Johnson and assigned to the present assignee, concerns a linearizer of the aforementioned type which includes an RF subassembly, an IF subassembly, and a processor subassembly. The RF subassembly includes a VCO which provides the FMCW signal to be transmitted. This subassembly also includes means for sampling the signal to be transmitted, delay means for delaying the sampled signal, modulating means for modulating the delayed signal, means for delaying the modulated signal, and means for mixing this twice-delayed signal with the output sample from the VCO.
In the IF subassembly, the signal from the mixing means is filtered and mixed with a sample of the signal which actuates the modulator to provide the beat signal between the sampled transmitter signal and the internally delayed signal. This beat signal is applied to a zero crossing detector to provide timing signals for use in the processor subassembly. The processor subassembly is used to generate a sweep drive voltage to be applied to the VCO. This subassembly contains means to measure the slope of the resulting VCO frequency sweep versus time curve and means to generate error signals as well as corresponding correction signals which can be combined with the intrinsic drive voltage so as to produce the desired linear frequency sweep voltage.
The present invention provides simplified and improved RF and IF subassemblies to operate signal processors of the type disclosed in the linearizer of Ser. No. 696,236.